In construction projects utilizing concrete structural members such as cast-in-place reinforced concrete beams or columns, it is important that the builder be able to ascertain the in situ compressive strength of the beams or columns. A common method for determining the strength of concrete structural members is the "pull-out" test in which a cylindrical metal test member having a stem projecting from one side is temporarily mounted on a concrete form so that it projects into the area to be filled with plastic concrete. The plastic concrete then envelopes the test member, and after the concrete is hardened, at the desired moment, tension is applied to the stem to induce pull-out failure in compression of the concrete. The force required to induce pull-out failure is related to the compression strength of the concrete.
A second test method is disclosed in my U.S. Pat. No. 4,425,801, issued Jan. 17, 1984. This apparatus consists of a generally cylindrical or disk-shape test member which is provided on its periphery with at least one notch so as to form a generally radially extending reactive surface which engages the adjacent concrete when the cylinder is rotated. A step attached to one face of the test member constitutes the means for applying a torque to the test member so as to engage the reactive surface with the adjacent concrete. In this latter method, the magnitude of the torque required to induce compressive failure in the hardened concrete is related to the compression strength of the concrete.
In both test methods, large aggregate particles encroaching on the anticipated rupture surfaces will also affect the accuracy of the test results. It is an object of this invention, therefore, to provide an improved method and apparatus for measuring the strength of hardened concrete in which a shield or screen is provided to prevent the encroachment of large aggregate particles in the portion of the concrete structure adjacent the reactive surfaces on the test members.